The rapid development of the mobile communications demands antennas having specific characteristics. Several kinds of antennas, such as antennas provided with dipole radiation elements or flat antennas employing so-called microstrip patch elements are known and widely used in applications related to mobile communications. The cell structure of the cellular mobile communications system is assumed to be known for a person skilled in the art and will not be described further here.
Generally, in an antenna comprising dipole antenna elements, several pairs of centrally fed dipole antenna elements are arranged on a panel forming the electrical ground plane. The antenna elements are fed with the signals to be radiated through a feed network. The antenna elements may be formed of a conductive material, for example brass or the like. The radio frequency signal is supplied through a port to the feed network which feeds the dipole elements. Alternating the line lengths of the feed network to each dipole element to generate phase delays is possible.
In an antenna employing the microstrip technique, the antenna generally comprises a number of antenna elements or patches over a ground plane and a distribution network. The distribution network can be realized using microstrip conductors in the same level as the radiating patches or on the other side of the ground plane. In the first case the conductors are simply connected to the sides of the patches. In the second case they are connected either galvanically with a separate conductor through a hole in the ground plane, so-called probe feeding or electromagnetically with coupling through an elongated resonant aperture in the ground plane, so-called aperture coupling. in some antenna designs the distribution network has two separate branches connecting two different polarizations to the antenna elements.
There are several important performance parameters, in particular for coverage of a sector in a cellular mobile communications system by means of base station antennas, for example a voltage standing wave ratio (VSWR), front-to-back radiation ratio and isolation between the polarization ports (in antennas using different polarizations). It is important that the radiation in rear direction of the antenna is maintained low towards the horizon, i.e. at elevation angle 0.degree., to reduce the level of interference in neighbouring cells and obtain high isolation. Generally, a high VSWR results in signal losses due to mismatch and a low isolation between the polarization ports, for example in a dual polarised antenna reduces the polarization diversity the gain and it will increase the filter requirements in the transmitted signal path of the base station.
In many installations the antennas are arranged to optimise the coverage, e.g. through high gain directed towards the cell edge, preferably very close to the horizon. In this case the back radiation, hereinafter called the rear beam, also has its maximum directed horizontally, which results in a relatively low front-to-back radiation ratio. In the radiating part of the antenna consisting of radiating element and feed network, it is easier to obtain low VSWR and higher isolation through the design and using electrical tilt, as the VSWR and coupling effects usually originate from the radiating elements.
Tilting the beam of an antenna, both electrically or mechanically to obtain certain features is known. For example U.S. Pat. No. 5,440,318 and Australian Patent No. 656857 (by the same inventors), describe arrangement of a panel antenna, particularly suitable for use in cellular communications system. The panel antenna, including bipolar radiating elements, comprises means to tilt the beam of the antenna downwards, both mechanically and electrically. The electrical tilting is mainly used for aesthetic reasons and secondly as a coarse method while the mechanical tilting is used as a fine method. These documents only discuss the down tilting of the beam.
U.S. Pat. No. 4,249,18 1 describes an arrangement to improve the average signal-to-interference ratio in at least one communication cell region by tilting the antenna gain pattern center-beam line of an antenna below the horizon. The antenna is tilted downwards by a predetermined amount. Antenna tilting is achieved either electrically or mechanically.
None of the above documents mention or show a method or arrangement for solving problems solved by the present invention. Even though, above Australian patent mentions an increased front-to-back ratio, this is achieved arranging the sidewalls of the panel negatively. Moreover, the up-tilting of the antenna beam is neither discussed nor shown in the prior art. The prior art solves specific problems which also may be solved through present invention, but they do not provide for any solutions for the problems solved by the present invention.